1. Field of the Invention
The present invention relates to the replication of information from a master record carrier onto a replicate material by optical contact printing and, more particularly, to apparatus and method useful for such replication.
2. Description Relative to the Prior Art
A master record carrier, for example a master videodisc, is a high quality recording of video program material. From the master disc, it is desirable to mass produce replicate discs for consumer use. One known method for replicating a videodisc uses a master disc comprised of a thin metal film evaporated onto an optically polished plate-glass disc. Video information is recorded in the master by using a laser beam to selectively melt the metal coating. The master is then coated with a photoresist material which is exposed through the rear of the glass disc. After processing, an array of bumps remains which coincides with the initial array of information holes. This bumpy surface is used to produce a metal stamper, or mold, by much the same techniques as are used in audio record manufacture. Discs are mass produced from the mold by thermoform replication, which generally takes one of three forms: (1) In compression molding, a soft "patty" of warm plastic is placed between the open halves of a mold cavity. The mold is heated and the halves are pressed together forcing the plastic to assume the shape of the cavity. (2) In injection molding, the halves of the mold cavity are first clamped shut, and then hot plastic is injected at high pressure to fill the cavity. (3) In embossing, a preformed sheet of plastic such as polyvinyl chloride is placed in a press and the metal stamper is pressed against the surface of the plastic sheet long enough to transfer the video information.
The above replication process can be simplified by using a photoresist master comprised of a glass disc blank coated with a thin, uniform layer of positive (or negative) photoresist. The photoresist is directly exposed with a laser recording beam, and processing produces an array of pits (or bumps) corresponding to the video information. A metal stamper, or mold, is made from the processed photoresist master and thermoform replication is used to produce replicate discs.
It is apparent that thermoform replication is a relatively complex technique, whether one uses a metal master or a photoresist master. An alternative--and attractive--replication process is optical contact printing. A conventional metal master is used, without modification by photoresist coating, etc., as an optical mask through which a contact print exposure is made. In general, however, optical contact printing is not well suited to the replication of videodiscs for two major reasons: (1) Video information recorded on a videodisc is commonly in the form of an array of elongated circular apertures only a few wavelengths of light in size; diffraction by such apertures causes contact between the master disc and the replicate material to be extremely critical. Even a slight separation occurring between the master disc and the replicate material during exposure will result in a large decrease in the fidelity of the recorded micro-imagery, the size of which should preferably be preserved to within a few percent, i.e., about 20 to 40 angstroms. (2) The standard videodisc is about 30 cm (twelve inches) in diameter and has literally billions of micrometer sized apertures covering the disc, thereby making it necessary to maintain uniform and intimate master-to-replicate contact over the entire image surface--a difficult problem to solve due to the presence of dust, dirt, disc imperfections, etc.
French Patent No. 2,310,586 (and corresponding German Offenlegungsschrift 2,620,283) discloses apparatus for replicating videodiscs onto a diazo material by optical contact printing. In accordance with that teaching, a web of replicate material comprising a diazoic substance on a support is unwound at constant speed. A transparent cylinder rotates on the replicate material as the replicate material is unwound. A flexible master record carrier having video information recorded in the form of spaced apertures is wrapped around the transparent cylinder. A monochromatic, ultraviolet lamp is disposed inside the cylinder along its longitudinal axis and emits radiation through the apertures of the master to expose the diazo layer of the replicate material. French Patent No. 2,310,586 apparently fails to recognize that the replication process is critically sensitive to the presence of foreign particles because there is no provision for cleaning the replicate material of such particles. In addition, the replicate material is at least partially wrapped around the cylinder to drive the cylinder and to contact the master record carrier during the contact print exposure. Such wrapping gives rise to compressive strain along the inside radius of the replicate material that will tend to distort the replicate material as well as causing the replicate material to abrade the master, thereby degrading the replicate image and also causing wear of the master record carrier.
Because of the exacting requirements for videodisc replication, designing and constructing a replication machine suitable for the mass production of replicate videodiscs by contact printing is a formidable problem. Such a machine would have to include apparatus for thoroughly cleaning the replicate material of virtually all foreign particles. Means would also have to be provided for rapidly bringing the master disc into close contact with the replicate material for successive contact print exposures. Further, it has been found that the contacting process must be highly uniform since variations in the intimacy of contact cause objectionable quality variations in the replicated video information. Still further, the replication of videodiscs by optical contact printing is critically sensitive to numerous factors, at least some of which are either not completely understood or are uncontrollable, thereby making the replication process somewhat unpredictable. It is, therefore, highly desirable that means be provided to enable an operator to remove a portion of the exposed replicate material for purposes of monitoring the quality of the replication process, preferably without disturbing the operating conditions in the replication machine.